Did somebody just make them up Esperanto style, or did elves get born speaking Sperethiel and Orks learn Or'zet when they goblinized? Either seems odd to me. If it's the former, why are they considered to be of cultural importance? If it's the latter, what kind of sense does that make?

Dunkelzhan left the orks a set of tablets from the fourth age that contained their racial language. Sperethiel's in game origin is a little murkier. Supposedly, and mysteriously rediscovered by the elven council of Tir Tariengeir (pardon the spelling, away from my books right now.)

Way back when, they thought it was an oh-so-cool idea to make the world of Earthdawn (a fantasy RPG) the past of the Earth in Shadowrun, so ever since, there have been all of these oblique references to Earthdawn scattered around the books. So Or'zet and Sperethiel are both actual ancient languages spoken by the orks and elves of Earthdawn. Or'zet was rediscovered in the tablets that Kesendeja mentions, while Sperethiel was preserved by the immortal elves, Gary-Stu GMPC's who don't age, and who were around during the Earthdawn setting.

The average man or woman on the street doesn't have this knowledge of a previous age, though, so a lot of people probably do assume that Or'zet and Sperethiel are made-up languages.

Yup. Dunkelzhan was sitting on the tablets and figured the orks could have them after he died. Why he didn't give them to the ork community before that, who knows. Their origin is the 4th world and Or'zet is what orks spoke back then.
The elves language is also a carry over from the 4th world and the various immortal elves, like Ehran the Scribe and Harlequin are the main source of that language. To cover up the fact they are immortals they just said it was 'discovered' in some lost archive and disseminated it through the two Tirs (Tairngire and NanOg).
Sperethiel has been around since the Tirs creation (mid 2020s), while Or'zet showed up after the Big D kicked the bucket in the mid 2050s.

I was always pretty sure Dunk intended to keep Or'zet a secret after seeing how badly the Tirs fucked up and tried to turn into separatist apartheid states, and decided in the end that if he wasn't going to be around they deserved to know.

Technically speaking, Sperethiel never really went away, since we have several survivors from the time when it was more widely spread that sill speak it fluently.
And at least in the irish elven land, there was some meddling, so probably from there . .

Sperethiel likely has been quite butchered since the dawning of the sixth age. Taken from the opening of Dawn Of The Artifacts: Dusk:

** “I think you’re jumping at shadows, old man,” the dwarf said, flicking the looted sword over to the black-haired ork, who caught it easily. A new wave of slender elves emerged from the citadel, swords
raised high, screaming horribly mangled Sperethiel war cries. The ork held his sword in a grip identical to the dwarf, mirroring his motion as they parried and slashed. **

And for those who might not know, the dwarf in this is Harlequin and the ork is Ehran The Scribe... I think. Might be the other way around, as I understand the two take to reversing who's what when they have their little chats from time to time.

Now I kinda wonder how Or'zet fared without it being a spoken language for about 5000 years.

As well as Ancient Egyptian or Mesopotamian perhaps.

Linguistically, I personally always considered Sperethiel something like pre-Sanskrit - the language all indo-european languages derive from, spoken by the historical Aryans. Evidence is relatively thin, of course, mostly because of Harlequin's reacion to the term Elf Nazi. However, the entire Earthdawn Barsaive setting (Ukraine, Russia) makes a lot more sense when considered like this, as this is where the historical Aryans are believed to have originated. Or'Szet is, I think, an offshoot of Sperethiel, or 2nd World pre-Sperethiel. Both are prehistoric languages from the Fourth World.

Cultural relevance in the Sixth World derives from metahuman nationalism mostly - the desire to be amongst themselves and not the White human majority who was font of burning them at the stake. Elves had the Immortals who preserved fragments of older Elf cultures and jump-started their movement. Orks got the Or'Szet code thanks to Dunkelzahn and followed suit, albeit with less success (though they never had failing White Human Nationalist empires to take advantage of and hence, had it harder than the Elves).

At least a few of those should also speak Or'zet, although they are probably unwilling to teach it.

In Earthdawn Or'zet is based on an older language, Or'zat, and strongly influenced by the Throalic language or least by the adaptation of its pronounciation to speakers lacking orkish speech organs (i.e. tusks). I'm not aware that either Or'zet or the Dwarven languages Throalic is derived from are based on Sperethiel. The supposed precursor of Sperethiel was used "in the days before Alachia's reign", so still during the Fourth World.

Alachia is a Second World creature, if I am not mistaken. However, ou're right. Or'zet is not a Sperethiel offshoot.